Geologists and engineers often evaluate subterranean formations for the purpose of improving hydrocarbon recovery. Once a formation of interest is located, one way of studying the formation is by obtaining and analyzing representative samples of rock. The representative samples are generally cored from the formation using a core drill or other core capture device. Formation samples obtained by this method are generally referred to as core samples. Analysis of core samples is generally regarded as the most accurate method for evaluating the characteristics of a formation and how the reservoir fluids (e.g. oil, brine, and gas) interact therein. Although many types of core sampling exist (e.g. rotary and percussion side-wall coring, cuttings, etc), being able to cut a conventional whole core provides the largest amount of core leading to the largest plug samples to test (improved accuracy with improved plug pore volume, etc.) and the largest continuous resource for geologic analysis.
Once the core sample has been transported to the surface, the core sample is analyzed to evaluate the reservoir characteristics, such as porosity, permeability, relative permeability, capillary pressure, wettability, lithology, etc. The analysis of the core sample is then used to plan and implement a well completion and production strategy and design. For example, analysis of core samples may reveal information useful for determining from which intervals to produce hydrocarbons or which intervals to stimulate or otherwise treat.
Unconsolidated and friable formations present significant challenges to recovering undamaged or useful core samples. Unconsolidated material is material with insufficient cementing agents between the grains to stop movement of individual grains during coring or handling, having compressive strengths less than about 10 psi. In other words, the term “unconsolidated” refers to loose or not stratified grains such as is the case with uncompacted, free flowing sand. Friable material, on the other hand, refers to material that is easily broken into small fragments or reduced to individual sand grains.
A common problem shared by unconsolidated and/or friable formations is the susceptibility of these formations to wash away from the mud flow at the coring bit or jam within the core liner during the core capture and retrieval process. In some cases, the core sample may not possess sufficient compressive strength to support the weight of the column of core sample already captured, or the core sample may simply fluidize or “wash out” during the core drilling process. Whatever the mechanism of core loss, core loss remains a significant problem in unconsolidated and friable formations. This problem is so significant that in many cases, no useful core sample is retrieved due to the severity of problems encountered while capturing and retrieving core samples. Indeed, it has been estimated that approximately 20% of core samples are lost in the United States and Canada due to the inability to core and or core damage.
The problem of obtaining useful cores is further exacerbated in deviated and horizontal wells due to the fact that as the deviation of a wellbore increases, the core becomes less self-supporting and more susceptible to inner tube friction and vibrations during entry.
Various conventional solutions have been proposed to mitigate the problem of core sample recovery and damage. In particular, many mechanical solutions have been proposed such as using low invasion coring bits to reduce the probability of fluidizing the core. This mechanical enhancement has enjoyed limited success in truly unconsolidated or friable formations.
In some cases, operators have resorted to capturing and recovering core samples in short segments to avoid core collapse. Even when this technique happens to work, however, it is expensive and costly in terms of the extra well bore trips required to sequentially recover the multiple core samples. With daily rig rates varying from $20,000 to $1 million dollars, any increase in time spent capturing and recovering core samples can be prohibitively expensive.
Additionally, a variety of remedial measures exist to mitigate the adverse effects of core damage. As one might imagine, however, remedial measures are far less effective at mitigating the adverse effects of core damage than successful preventative measures.
Accordingly, there is a need for enhanced core capture and recovery methods that address one or more of the disadvantages of the prior art.